This invention relates to a noble metal catalyst, and more particularly to a boron nitride (BN) supported noble metal catalyst.
Volatile organic compound (VOC) is a low boiling-point hydrocarbon and is one of primary air pollutants. The sources of VOC include the exhaust gas from the automobiles, the gasoline vapor from the gas station, the organic solvent in the printing shops, and so on. It""s well known that VOCs, such as methanol, benzene and gasoline, are harmful to the human health. Catalytic combustion is one of effective methods to eliminate VOC in air via the deep oxidation to water and carbon dioxide. Conventionally, supported noble metal catalyst using oxides as supporting material, e.g. supported Pt gamma-alumina, is often used in the deep oxidation of VOC (J. J. Spivey, Ind. Eng. Chem. Res., 26, 2165-2180, 1987).
In general, a high temperature in the range of 200xc2x0 C. to 500xc2x0 C. is required for complete combustion of VOC to occur even with the help of noble metal catalyst, such as Pt/Al2O3. This can be costly as extra fuel and lengthy induction time are often required to bring the VOC stream to the desired reaction temperature. The advantage of low oxidation temperature can reduce fuel consumption particularly for large volumes of dilute VOC polluted air as well as shorten the induction time by low light-off temperature. Traditional metal oxide supports, such as Al2O3, SiO2, and zeolite are insulating material and possess rather low thermal conductivity, as well as facile phase transformation to another crystalline phase under water vapor at high temperature. Accumulation of exothermic reaction heat may be severe on those hot spots and lead to the deactivation of catalyst activity during deep oxidation of VOC. The property of support also plays an important role in supported noble metal catalysts. A metal-support interaction generally exists in most oxide-supported metal catalysts and brings about a negative influence on the catalytic activity. Therefore, one way to avoid these disadvantages would be the use of a non-oxide material as the support.
The graphite-like hexagonal boron nitride (BN) is the most stable isomer of BN under ambient conditions. It exhibits high thermal conductivity, thermal stability, acid-base resistance, oxidation resistance, and appropriate mechanical strength. Furthermore, BN is hydrophobic preventing moisture condensation on its surface (K. Niedenzu and J. W. Dawson, Boron Nitride, Boron-Nitrogen Compounds, Chapter 6, Springer-Verlag, Berling, 1965).
As known in the prior arts, boron nitride was reported as catalyst materials in several applications. Pieters et al. (U.S. Pat. No. 4,060,499) described a catalyst consisting cuprous chloride intercalated in crystalline BN for substituted chlorination reaction. Monnier and Muehlbauer (U.S. Pat. No. 4,950,773) disclosed a selective epoxidation of olefins using supported Ag catalyst, in which BN was one of inorganic supporting materials. Yoneda et al.(U.S. Pat. No. 4,956,326) reported a Pt catalyst comprising a ceramic supports, such as carbides, nitrides and oxides for dehalogenation of a halide. Submicron BN particle was used as an activating catalyst in the formation of polysiloxane resins in the U.S. Pat. No. 6,183,873. Jenkins (U.S. Pat. No. 4,897,253) reported a catalytic process for the generation of hydrogen via the partial oxidation of hydrocarbons using a platinum and chromium oxide supported on silica, and suggested other refractory, such as carbide and nitride, could be one option of supporting materials. German researchers von Hippel et al. (U.S. Pat. No. 5,928,984) developed a process for preparing catalytically active coatings for the synthesis of hydrogen cyanide on molded items consisting of Pt and aluminum oxides, which could also be replaced by nitrides, such as BN. Acres and Darling (U.S. Pat. No. 4,163,736) disclosed a method to coat a refractory compound with a magnesia barrier layer and Pt group metal. The refractory core was selected from oxides, carbides or nitride. The refractory compound can be used under non-oxidizing or reducing reaction condition at high temperature. Jacobsen reported that the ammonia synthesis reaction can be performed using a barium-promoted BN-supported ruthenium catalyst (C. J. H. Jacobsen, J. Catal. 200, 1-3, 2001).
However, none of the above prior arts is applied in the deep oxidation of organic compounds in that the advantages of BN as a support in the oxidation reaction is most desirable. The present invention discloses a novel BN supported Pt group metal catalyst for the deep oxidation of organic compounds.
It is an object of the present invention to provide a boron nitride supported noble metal catalyst for deep oxidation of organic compounds.
In accordance with an aspect of the present invention, BN can be purchased from a commercial market with a specific surface area ranging from 1 to 100 m2/g. Precursor salt, contained noble metal is dissolved in an organic solvent resulting in a noble metal/solvent solution. The BN supported noble metal catalyst is prepared by the so called xe2x80x9cincipient wetness techniquexe2x80x9d to disperse the noble metal on the surface of BN. In this technique, the powder BN is soaked with a minimum volume of noble metal-solvent solution which is just sufficient to be fully absorbed by the given amount of BN. A loading of noble metal is preferably ranging from 0.1 to 5.0 wt % of the catalyst. The BN supported noble metal catalyst is dried at ambient temperature, then thermal treated at 300-400xc2x0 C. for one hour, under a flow of nitrogen, air, oxygen or hydrogen/nitrogen mixture.
The superior activity of BN supported noble metal catalyst is demonstrated by the extreme low light-off temperatures in the deep oxidation reaction of organic compounds under air stream. The concentration of organic compounds ranges from 100 to 20 vol % at volume hourly space velocity (VHSV) of 2000-40000 hrxe2x88x921 and temperature in the range of 0 to 600xc2x0 C. In conclusion, Pt/BN outperforms the traditional oxide-supported Pt catalysts with respect to the life and activity.
The above objects and advantages of the present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which: